Fatigue-resistance sheet slitting method and resulting sheet

ABSTRACT

A sheet of material ( 111 ) having a plurality of bend-inducing structures ( 113 ) configured and positioned to produce bending along a bend line ( 115 ). The bend-inducing structures ( 113 ) have arcuate return portions ( 122 ) extending from opposite ends ( 121 ) back along the bend-inducing structures ( 113 ) toward the other return portion ( 122 ) and each return portion ( 122 ) has a length dimension and a radius of curvature reducing stress concentrations. Preferably, the length dimension of the arcuate return portion ( 122 ) is in excess of twice the thickness. The lateral distance, LD, to which the bend-inducing structures ( 113 ) is formed in the sheet away from the bend line ( 115 ) is preferably minimized by small radius arcs ( 125 ) which connect the return portions ( 122 ) to the remainder of the bend-inducing structures ( 113 ). A method of forming a structure ( 131 ) from a sheet of material ( 111 ) to resist cyclical loading is also disclosed, as is a method to increase the fatigue resistance of a structure ( 131 ) formed by bending a sheet of material ( 111 ) along a bend line ( 115 ) having a plurality of bend-inducing structures ( 113 ).

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 60/587,470, filed Jul. 12, 2004, entitled METHOD FOR INCREASING THEFATIGUE RESISTANCE OF STRUCTURES FORMED BY BENDING SLIT SHEET MATERIALAND PRODUCTS RESULTING THEREFROM, the entire contents of which isincorporated herein by this reference.

This application is also a Continuation-in-Part Application of U.S.patent application Ser. No. 10/672,766, filed Sep. 26, 2003, andentitled TECHNIQUES FOR DESIGNING AND MANUFACTURING PRECISION-FOLDED,HIGH STRENGTH, FATIGUE-RESISTANT STRUCTURES AND SHEET THEREFOR, which isa Continuation-in-Part Application of U.S. patent application Ser. No.10/256,870, filed Sep. 26, 2002, and entitled METHOD FOR PRECISIONBENDING OF SHEET OF MATERIALS, SLIT SHEETS FABRICATION PROCESS, now U.S.Pat. No. 6,877,349, which is a Continuation-in-Part Application of U.S.patent application Ser. No. 09/640,267, filed Aug. 17, 2000, andentitled METHOD FOR PRECISION BENDING OF A SHEET OF MATERIAL AND SLITSHEET THEREFOR, now U.S. Pat. No. 6,481,259, the entire contents ofwhich is incorporated herein by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, in general, to the bending of sheets ofmaterial having bend-inducing structures formed therein, such as slits,grooves, perforations or steps, and more particularly, relates toimproving the resistance of structures formed by bending such sheets tofatigue failure during cyclical loading.

2. Description of Related Art

A commonly encountered problem in connection with bending sheet materialusing conventional sheet bending equipment, such as a press brake, isthat the locations of the bends are difficult to control because ofbending tolerance variations and the accumulation of tolerance errors.For example, sheet metal may be bent along a first bend line withincertain tolerances. A second bend, however, often is located based uponthe first bend, and accordingly, the tolerance errors can accumulate.Since there can be three or more bends which are involved to create anenclosure or closed structure, the effect of cumulative tolerance errorsin conventional prior art bending techniques can be significant.

One approach to this problem has been to try to control the location ofbends in sheet material through the use of bend-inducing orbend-controlling structures, such as slits, grooves, perforations or thelike. Bend-inducing structures can be formed in sheet stock at veryprecise locations, for example, by the use of computer numericallycontrolled (CNC) devices to manipulate lasers, water jets, punchpresses, knives or even single point tools.

Slits, grooves, perforations, dimples and score lines have been used invarious patented systems as bend-inducing or producing structures forbending sheet material. U.S. Pat. No. 6,640,605 to Gitlin et al. employsparallel offset slits to create bendable sheets in which connectingtwisted straps or “stitches” span across the bend line. The Gitlin etal. slitting technique was developed to achieve decorative affects, andthe resulting bends were reinforced in most applications to provide thenecessary structural strength. U. S. Pat. No. 5,225,799 to West et aluses a grooving-based technique to fold up a sheet of material to form amicrowave wave guide or filter. In U.S. Pat. No. 4,628,661 to St. Louis,score lines and dimples are used to fold metal sheets. In U.S. Pat. No.6,210,037 to Brandon, slots and perforations are used to bend plastics.The bending of corrugated cardboard using slits or die cuts is shown inU.S. Pat. No. 6,132,349 and PCT Publication WO 97/24221 to Yokoyama, andU.S. Pat. No. 3,756,499 to Grebel et al. and U.S. Pat. No. 3,258,380 toFischer, et al. Bending of paperboard sheets also has been facilitatedby slitting, as is shown in U.S. Pat. No. 5,692,672 to Hunt, U.S. Pat.No. 3,963,170 to Wood and U.S. Pat. No. 975,121 to Carter.

In most of these prior art sheet bending systems, however, thebend-inducing structures greatly weaken the resulting structure, or thebend-inducing structures do not produce the desired precision in thelocation of the bends, or both.

The problems of precision bending and retention of strength are muchmore substantial when bending metal sheets, and particularly metalsheets of substantial thickness. In many applications it is highlydesirable to be able to bend metal sheets with low force, for example byhand, with using only hand tools or with only moderately powered tools.

Well known conventional fabrication techniques for producing rigidthree-dimension structures include the joining together of sheetmaterial by jigging and welding, or clamping and adhesive bonding, ormachining and using fasteners. In the case of welding, problems arise inthe accurate cutting and positioning of the individual pieces duringwelding, and the labor required to manipulate a large number of parts issignificant, as are the quality control and certification burden.Additionally, welding has potential problems in connection withdimensional stability caused by the heat affected zone of the weld.

Welding of metal sheets or plates having significant material thicknessis often achieved using parts having beveled edges made by grinding orsingle point tools. This adds significantly to the fabrication time andcost. Moreover, fatigue failure of heat affected metals under cyclicalloading is a problem for joints whose load bearing geometries are basedupon welding, brazing or soldering.

A new system for precise bending of sheet material, including thicksheets, has been devised in which improved bend-inducing orbend-controlling structures are employed. The bend-inducing structuresare configured and positioned in a manner such that thethree-dimensional structure resulting upon bending of the sheet hassubstantially improved strength and dimensional precision as compared toprior art slitting techniques, such as, for example, are disclosed inthe Gitlin et al. U.S. Pat. No. 6,640,605. The position andconfiguration of these new and improved bend-inducing structuresfacilitate bending of the sheet precisely along the bend line, mostpreferably by causing edge-to-face engagement of the sheet material onopposite sides of the bend-inducing structures during the entire bendfor control of the bend location.

The configurations and positioning of these new and improvedbend-inducing slits, grooves and steps are described in much more detailin the above set forth Related Applications, which are herebyincorporated by reference in their entireties into this application.

Using the improved bend-inducing structures for bending sheet materialhas many advantages, not the least of which is the ability to use aseries of precisely located bends to close the sheet of material backupon itself during bending, for example, in order to fabricate a boxbeam. Press brake bending, by contrast, is not well suited to formclosed structures such as box beams. Box beams are exemplary ofstructures that have many applications and have heretofore been formedmore traditionally by welding together of metal sheets or plates, ratherthan by bending of a single sheet or plate into a closed hollow beamstructure.

Bending sheet material to form a box beam has substantial cost-savingadvantages over fabrication of the beam by welding, if the resultantbeam has substantially the same strength, and if it does not failprematurely due to fatigue during the cyclical loading. When a box beamis loaded during use, it typically will be loaded transversely to itslength, that is, transversely to the longitudinally extending corners ofthe beam along which the sheets or plates are welded together, or in thecase of a folded single sheet, along the longitudinally extending bendlines. Such loading is often cyclical and results in fatiguing of thebeam at its corners. For welded box beams, therefore, fatigue failuretypically occurs along the welded corners, and if a bent sheet is to beused, the corner bend lines will also be the area most likely to fail.

Accordingly, it is an object of the present invention to provide amethod for increasing the fatigue resistance of structures formed bybending slit sheet material.

It is another object of the present invention to provide an improvedconfiguration of a bend-inducing structure for sheet material that willsubstantially improve the fatigue resistance of three-dimensional objectformed by bending the sheet material.

A further object of the present invention is to provide increasedfatigue resistance in bent sheet material and improve strength at thebend line of the sheet material.

Still a further object of the present invention is to provide a methodand apparatus for enhancing the fatigue resistance of bent, slit sheetmaterial which does not undesirably increase the fabrication costs, canbe applied to a wide range of structures, and is adaptable for use withsheets of various thicknesses and types of materials.

The method and apparatus of the present invention have other objects andfeatures of advantage which will become apparent from, or are set forthin more detail in, the accompanying drawing and the followingdescription of the Best Mode of Carrying Out the Invention.

SUMMARY OF THE INVENTION

In one aspect, the present invention is comprised of a sheet of materialformed for bending along a bend line and having a plurality ofbend-inducing structures configured and positioned to produce bendingalong the bend line. At least one of the bend-inducing structures, andpreferably all of them, have arcuate return portions extending fromopposite ends of the bend-inducing structure and returning along thebend-inducing structure toward the other return portion. The returnportions each are configured to significantly increase resistance tofatigue resulting from cyclical loads oriented in a direction transverseto the bend line by having arcuate lengths and radii reducing stressconcentrations. The bend-inducing structures preferably are slits,grooves or steps which are configured to produce edge-to-face engagementon opposite sides of the bend-inducing structures during bending. Stressconcentrations can be reduced by forming the arcuate return portionswith a cord length at least approximately twice the thickness dimensionof the sheet of material. The arcuate return portions further preferablyhave chords oriented substantially parallel to the bend line, and aradii of curvature of the return portions which are at leastapproximately three times the thickness dimension of the sheet ofmaterial.

In another aspect of the present invention a method of increasing thefatigue resistance of a structure formed by bending a sheet of materialalong a bend line having a plurality of bend-inducing structures isprovided. The method comprises, briefly, the step of forming thebend-inducing structures to extend along the bend line and have arcuatereturn portions extending from opposite ends of the bend-inducingstructures back along the bend-inducing structures toward the otherreturn portion. The return portions have a length dimension along thebend line and a radius of curvature selected to be sufficiently large tosignificantly increase resistance to fatigue upon cyclical loading ofthe structures transverse to the bend line.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a sheet of material having bend-inducingstructures formed therein as shown in the Related Applications.

FIG. 2 is a top plan, schematic representation of the slits of FIG. 1,and FIG. 2A is an enlarged, top plan view of the ends of the slits ofFIG. 2.

FIG. 3 is a top plan, schematic representation corresponding to FIG. 2of an alternative embodiment of the slits showing arcuate returnportions.

FIG. 3A is an enlarged, top plan view of an end of the slit of FIG. 3.

FIG. 4 is a top plan, schematic representation corresponding to FIG. 2of a further alternative embodiment of the slits showing an extendedarcuate return portions.

FIGS. 4A and 4B are enlarged, top plan views of the end of the slit ofFIG. 4.

FIG. 5 is a top plan, schematic representation corresponding to FIG. 2of a further alternative embodiment of slits having a configuration andconstructed in accordance with the present invention.

FIGS. 5A and 5B are enlarged, top plan views of the end of the slit ofFIG. 5.

FIG. 6 is a schematic, side elevation view of a fatigue test stand witha box beam constructed using the slit configurations of FIG. 4 inposition for testing.

FIG. 6A is an end view of the beam of FIG. 6.

FIG. 7 is a graph of stress versus cycles-to-failure for beams testedusing the fatigue test stand of FIG. 6 and showing welding curves forclass B to class G welds.

FIG. 8 is a table showing the test results for the beams tested usingthe test stand of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

Reference will be made in detail to the preferred embodiment of theinvention, an example of which is illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiment, it will be understood that it is not intended tolimit the invention to that embodiment. On the contrary, the inventionis intended to cover alternatives, modifications and equivalents, whichmay be included within the spirit and scope of the invention as definedby the appended claims.

The present method and apparatus for precision bending of sheet materialis based upon the bend-inducing slits, grooves or steps disclosed in theabove-identified Related Applications, and particularly, as disclosed inU.S. patent application Ser. No. 10/672,766, filed Sep. 26, 2003 andentitled TECHNIQUES FOR DESIGNING AND MANUFACTURING PRECISION-FOLDED,HIGH STRENGTH, FATIGUE-RESISTANT STRUCTURES AND SHEETS THEREFOR. FIG. 6of U.S. patent application Ser. No. 10/672,766 has been incorporated inthis application as FIG. 1 to illustrate the changes made by the presentinvention to the slit groove or step configurations in order to increasefatigue resistance.

Referring specifically to FIG. 1, a sheet of material 41 to be bent orfolded along a bend line 45 is formed with a plurality of longitudinallyextending bend-inducing structures. These bend-inducing structures maybe any one of slits, grooves or steps 43 positioned along bend line 45,but for brevity they will be referred to herein as “slits” or“bend-inducing structures.” Each bend-inducing structure 43 is shown ashaving a kerf in FIG. 1 and essentially no kerf in FIGS. 2 through 5B.The presence or absence of a kerf does not form a part of the presentinvention. Slits 43 also have enlarged stress-relieving end openings 49,or a curved end section 49 a (the slit on the right-hand end of FIG. 1).In addition, the slits may have a curved end. Curved end 49 a terminatesthe slits in a relatively low stress zone, thereby decreasing thelikelihood that cracking will initiate at a terminus of the curved end.Slits 43 are configured in a manner producing bending and twisting ofobliquely oriented bending straps 47 about a virtual fulcrumsuperimposed on bend line 45. The configuration and positioning of thebend-inducing structures, including selection of the jog distance andkerf width, causes the sheet material on opposite sides of thebend-inducing structures to tuck or to move into an edge-to-faceinterengaged relationship during bending, as is set forth in detail inthe Related Applications and will not be repeated herein. Mostpreferably, edge-to-face interengagement occurs throughout the bend toits completion; but, the jog distance and kerf also can be selected toproduce edge-to-face interengagement only at the start of the bend,which also will tend to ensure precise bending. Thus, as used herein,the expression “during bending” is meant to include edge-to-faceinterengagement at any stage of the bend that will produce precisebending. Interengagement only at the end of the bend will not controlthe location of the bend with the same degree of precision.

As shown in FIG. 1, pairs of elongated slits 43 are preferablypositioned on opposite sides, of and proximate to, bend line 45 so thatpairs of longitudinally adjacent slit end 51 on opposite sides of thebend line define a bending web, spline or strap 47, which can be seen toextend obliquely across bend line 45. “Oblique” and “obliquely,” shallmean that the longitudinal central axis of straps 47 cross the bendline, and cross at an angle other than 90 degrees. Thus, each slit,groove or step end portion 51 diverges away from bend line 45 so thatthe center line of the strap is skewed or oblique to the bend line. Thisproduces bending as well as twisting of the strap.

Unlike the slits or grooves of the prior art Gitlin, et al. U.S. Pat.No. 6,640,605, which are parallel to the bend line in the area definingthe bending straps, the divergence of the bend-inducing structures 43from bend line 45 results in oblique bending straps that do not requirethe extreme twisting present in the straps of the Gitlin, et al. patent.Moreover, the divergence of bend-inducing structures 43 from bend line45 results in the width dimension of the straps increasing as the strapsconnect with the remainder of sheet 41. This increasing width enhancesthe transfer of loading across the bend so as to reduce stressconcentrations and to increase fatigue resistance of the straps.

As above noted, the width or kerf of slits and the transverse jogdistance across the bend line between slits, are preferably dimensionedto produce interengagement of sheet material on opposite sides of theslits during bending. If the kerf width and jog distance are so largethat contact does not occur, the bent or folded sheet will still havesome of the improved strength and fatigue-resistance advantages ofoblique bending straps. In such instances, however, there are no actualfulcrums for controlled bending to occur so that bending along bend line45 becomes less predictable and precise. Similarly, if the strapdefining structures are grooves 43 which do not penetrate through thesheet of material, the grooves will define oblique, high-strengthbending straps, but edge-to-face sliding will not occur during bendingunless the groove is so deep as to break-through during bending andbecome a slit.

It is also possible for the slits 43 to actually be on the bend line oreven across the bend line (a negative jog distance) and still produceprecise bending from the balanced positioning of the actual fulcrumfaces 55 and the edges of lips 53 sliding therealong. A potentialdisadvantage of bend-inducing structures 43 being formed to cross thebend line 45 is that an air-gap would remain between the opposed edgesand faces. An air-gap, however, may be acceptable in order to facilitatesubsequent welding, brazing, soldering, adhesive filling or if anair-gap is desired for venting.

In the slit sheet of FIG. 1, both oblique bending straps 47 andstress-reducing opening or enlargements 49 have been employed in anattempt to increase the resistance to fatigue failure of the structureformed by bending sheet 41. Additionally, the right-hand slit or groove43 has been formed with an arcuate return portion or extension 49 a inorder to terminate slits 43 in a zone of relatively low stress. Whileeffective to some extent, these strategies for increasing the fatigueresistance of bend-inducing slits, grooves or steps sill have notachieved the fatigue resistance that is desirable for structures whichare subjected to repeated heavy cyclical loading.

More particularly, box beams which are formed using the sheet slitting,grooving or step-forming techniques as taught by the above-identifiedRelated Applications are often subjected to cyclical loading in bending.Such loading can cause premature fatigue failure of the beams, withdisastrous effects.

FIGS. 2, 2A, 3, 3A, 4, 4A and 4B schematically illustrate the evolutionof the configuration of the bend-inducing structures which have resultedin the greatly improved, fatigue-resistant geometry shown in FIGS. 5, 5Aand 5B.

FIGS. 2 and 2A correspond to FIG. 1 except that the bend-inducingstructures 43 are shown with ends 51 which do not have stress-relievingopenings or enlargements 49, as shown in FIG. 1. Similarly, ends 51 inFIGS. 2 and 2A do not have a return portion 49 a which curves back alongthe slits.

In FIGS. 2 and 2A, diverging slit ends 51 again define oblique bendingstraps 47, which will produce precise bending of sheet 41 along bendline 45. When the sheet of FIGS. 2 and 2A is bent and then loadedtransversely to bend line 45, failure of the resulting structure undercyclical loading will most likely occur at the ends of slits 43, asschematically shown in broken lines at 39 in FIG. 2A. Crack 39 willpropagate transversely away from bend line 45 and can cause failure ofthe three-dimensional structure formed by bending sheet 41.

In FIGS. 3 and 3A, sheet 71 is formed with a plurality of bend-inducingstructures, such as slits 73, which are positioned relative to bend line75 in a manner taught by the Related Applications. In the embodimentshown in FIGS. 3 and 3A, end portions 81 of the slits are formed withrelatively large diameter arcuate return portions 82. Thus, the returnportions 82 are similar in concept to that shown in FIG. 1 by arcuateend 49 a, but the radius of curvature of end return portions 82 is muchgreater than was the case for return portion 49 a. Again, the concept isto bring any stress-increasing crack tips to a low stress zone so thatcracks do not initiate from the tips.

It was discovered, however, that when a three-dimensional structure wasformed by bending sheet 71 along bend line 75, and thereafter thestructure was loaded transversely to bend line 75, fatigue failure didnot occur at end 83 of return portion 82, but instead, occurred, asshown by broken line 69, proximate point 84 of return portion 82 whichis farthest away from bend line 75.

In an effort to attempt to avoid the stress concentration resulting fromthe configuration of arcuate return portion 82, sheet 91 in FIGS. 4, 4Aand 4B was formed with bend-inducing slits 93 along bend line 95 As bestmay be seen in FIGS. 4A and 4B, the bend-inducing structures are formedwith return portions 102 which flatten out or have relatively largerradii of curvature in the area which failure might occur. The returnportions then hook back in at 103, again to attempt to avoid stressconcentration at the end of the bend-inducing structures. When bentalong bend line 95 and then transversely loaded, however, cracking againoccurred upon failure of the structure at crack 89, shown by a brokenline in FIGS. 4A and 4B. This cracking occurred at 104, which isapproximately the position which is furthest from bend line 95.

FIGS. 5, 5A and 5B illustrate the configuration bend-inducing slits,grooves or steps which have been found to have substantially increasedresistance to fatigue failure. This configuration is also shown in priorU.S. patent application Ser. No. 10/672,766 as FIG. 11.

In FIG. 5, a sheet of material 111 has been slit, grooved or steppedwith bend-inducing structures 113 along bend line 115 in a manner as setforth in the above-identified Related Applications. The bend-inducingstructures 113 are generally continuous compound arcuate shapes and haveend portions 121 which define bending straps 117 that extend obliquelyacross bend line 115 in a manner also described above and in the RelatedApplications. Arcuate return portions 122 are provided on opposite ends121 of bend-inducing slits 113, with ends 121 being connected to returnportions 122 by relatively smaller diameter arcs 125. Each returnportion 122 returns along bend line toward the other return portion.Finally, the return portions most preferably include ends 123 which hookor extend back toward bend line 115.

As will be seen from the Examples set forth hereinafter, a dramaticimprovement in the fatigue resistance of the bent structures formedusing the slit configuration of FIG. 5 over that of FIG. 4, and overthat of commercially available welding, has been experienced.

Comparing the slits of FIGS. 4 and 5, the dramatic increase inresistance to fatigue is believed to reside in one or more of thefollowing factors. First, the length of the arcuate return portion 102in FIG. 4A can be seen to be substantially shorter than the length ofthe arcuate return portion 122 in FIG. 5A. The ends of the slits in FIG.4 are continuous curves which transition from end radius 105 to returnradius 102 and then to the terminal radius 103. The arc angle of thereturn 102 for the FIG. 4 slits was only 3.7 degrees. The arc angle forthe slits of FIG. 5, by contrast, was 26.7 degrees. Thus, the chordsubtended by arc 122 in FIG. 5A is much longer than the chord in FIG.4A. This is believed to be very important in avoiding stress riserswhich will produce fatigue failure.

Another way of expressing this increased return length is that returnportions 122 extend along the slit by a much greater percentage of theslit length than is the case for return portions 102. Thus, the chordlengths of return portions 122 are on the order of about 20% of theoverall slit length in the FIG. 5 configuration, while they are onlyabout 4% of the slit length in the FIG. 4 configuration. Mostpreferably, and as is the case in both configurations, the returnportion chords are substantially parallel to the bend lines 95 and 115,respectively.

The radius of return portion 102 in FIG. 4B, however, is actually longerthan the radius of curvature of return portion 122 in FIG. 5B. Theradius of curvature of return 102 in FIG. 4B is 4.32 times the thicknessof the sheet of material, which was 0.125 inches in this case. In FIG.5B, the radius of curvature of return portion 122 can be seen to be only3.161 times the thickness dimension of the sheet of material, also 0.125inches. While it is believed that the radius of curvature of the returnportion should not be too small so as to arc away from the bend line 115in a manner which provides a site for stress risers, over a level, whichis not yet known, there is believed to be a reasonable amount oflatitude with respect to the radius of curvature of the return portion.

As will also be seen from FIGS. 4B and 5B, the radius of curvature ofend portion 125 is less than the radius of curvature of end portion 105.Thus, a radius of 0.124 times the thickness dimension of the sheet ofmaterial is employed in the slits of FIG. 5B, while a radius of 0.468times the thickness dimension of the sheet of material is employed inthe slits of FIG. 4B. The lateral distance, LD, to position 104 in FIG.4 from bend line 95 is significantly greater than the lateral distance,LD, of the equivalent position in the geometry of FIG. 5B.

Minimizing the lateral distance to which the slits extend away from thebend line is thought to be important because the slits cut into thenative material on either side of the bend line. When the beam is loadedas shown in FIG. 6, the bottom side 143 of the beam will be undertension so that a band of native material just above the slits will becalled upon to resist the tension forces along the length of the beam.As the arcuate slits have an end radius 105 which increases, the band ofunbroken native material moves away from the bend line by the lateraldistance, LD (see FIG. 5B), subjecting it to more stress in resistingthe tension loading forces.

At this point, sufficient testing has not been conducted in order togenerate complete curves as to the effects of return portion arc angles,return portion radii, or end arc radii (lateral distances into thenative material) so as to demonstrate where the substantially enhancedfatigue resistance begins to be significant. It is believed that theseare likely to be continuous curves with the arc angle of the returnportion being the most critical factor. It is also believed that theconfiguration of FIG. 5 will scale off of the thickness dimension of thesheet of material. Since the improvement in fatigue resistance allows abeam to be folded from sheet material and have a fatigue resistance manytimes that which can be achieved in welded equivalent structures, theexact point at which the performance exceeds a welded structure'sperformance may tend to be somewhat academic. Suffice it to say that theconfiguration of FIGS. 5, 5A and 5B will substantially out perform boxbeams which are welded together from plate material in fatigueresistance.

EXAMPLES

FIG. 6 schematically illustrates a box beam as positioned on a fatiguetest stand. The box beams tested each had a square cross-section with adimension of 4 inches on each side and included a flange 132 which wasfolded inside one of the sidewalls and secured thereto by fastenerassemblies 133, in this case a bolt and nut. The fasteners were placedevery 4 inches along the length of the beam, and beam 131 had an overalllength of 48 inches. A support assembly 135 was provided proximate eachend of beam 131, and forced distributing plates 137 used to avoid localconcentrations of stress at support stands 135.

Beam 131 was loaded at two locations 139 on either side of the center ofthe beam. The loads were spaced from each other by a distance ofapproximately 6 inches. Again, load distributing plates were employed at139, and arrows 141 schematically illustrate that the beam was loadedfrom a minimum load up to a maximum load. Loading was cycled betweenminimum and maximum load until beam failure occurred. As will be seenfrom FIGS. 6 and 6A, therefore, a bottom side 143 of the beam wascycling in tension, while a top side 145 was compressed under thetransverse bending load of the beam. In each case, failures occurredalong bottom side 143 of the beam with cracks propagating upwardly fromside 143 towards side 145.

FIG. 7 shows the test results for various beams which were tested usingthe test stand of FIG. 6. The stress was measured in Mega-Pascals,(MPa), and has been plotted versus Cycles to Failure. Also, shown onFIG. 7 are the Cycles to Failure curves for welded box beams, as afunction of the class of the weld. Thus, a class B weld is shown as thetop curve, while a class G weld is the bottom curve. The datarepresented by the “class B weld” to “class G weld” curves was generatedtesting “class B weld” through “class C weld” steel box beams, whichbeams are welded at the corners using the various welding classstandards, which are known in the industry. Typically, commerciallyavailable box beams will be welded at the level of a class F weld.

The data points on FIG. 7 were for two types of box beams, namely oneseries using the slits of FIG. 4 and the other series using the slits ofFIG. 5. When the initial tests were run, the trial load range wasrelatively low, namely 17.5 (e.g., Stress Range of approximately 90-100MPa). Data points 161, 162, 163 and 164 were all run using the lowermagnitude of cyclical loading as a trial. The data points 161, 162 and163 are all for box beams formed using the slit of FIG. 4. The datapoint 164 is for a box beam having FIG. 5 slits and having a trial loadof 17.5 (e.g., Stress Range of approximately 100 MPa), but the beam didnot fail at data point 164.

It was decided that the load range should be increased for final testingand data points 171, 172, 173, 174 and 175 are for beams which wereloaded with a load range of 26 (e.g., Stress Range of approximately 150MPa). Data points 172, 173 and 174 are for box beams folded from sheetmaterial formed with slits having the configurations of FIG. 4 whiledata points 171 and 175 are for box beams which were folded from sheetsslit in accordance with FIG. 5.

Data point 171 is a relatively early failure which occurred in a FIG. 5box beam, not because the beam failed at any of the slits, but becausethe beam went out of square into a rhombus mode during cycling. Thisrhombus mode cycling resulted in a premature failure. Data points 164and 175 are for the same type of beam, namely a beam with FIG. 5 slits.The beam was cycled up to 2,100,000 cycles at the low trial load rangeof 17.5 (e.g., Stress Range of 100 MPa) and, since no failure occurred,the loading was then increased to 26 (e.g., Stress Range of 150 MPa).The beam loading was then continued up to 3,827,753 cycles, at whichpoint the test could not be completed because the failure occurred atone of the load points 139, indicating that failure was not purely afunction of the beam's characteristics but instead a function of thebeam/test configuration. Thus, the test essentially was not completed tofind the ultimate real limit of beams having FIG. 5 slits.

As will be seen, data point 175 is above the curve for a class C weld,much less that of a class F weld, the commercially available welds. Aclass F weld would fail, on average, at about 600,000 cycles at the loadrange of 26 (e.g., Stress Range of approximately 150 MPa). Thus, a bentor folded box beam using the slit configuration of FIG. 5 has more thansix times the cycling capacity of a commercially welded, class F, boxbeam, and the upper limit of the box beam of the present invention isstill not known.

FIG. 8 shows a table of the test results used to generate the data ofFIG. 7.

The foregoing descriptions of a specific embodiment of the presentinvention has been presented for the purpose of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiment was chosen and described in order to bestexplain the principles of the invention and its practical application tothereby enable others skilled in the art to best utilize the inventionand the embodiment with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

1. A sheet of material formed for bending along a bend line comprising:a sheet of material having a plurality of bend-inducing structuresconfigured and positioned to produce bending along the bend line, atleast one bend-inducing structure having arcuate return portionsextending from opposite ends of the bend-inducing structure andreturning along the bend-inducing structure toward the other returnportion, the return portions each being configured to significantlyreduce stress concentrations resulting from loads oriented in adirection transverse to the bend line.
 2. The sheet of material asdefined in claim 1 wherein, the bend-inducing structures are one ofslits, grooves and steps.
 3. The sheet of material as defined in claim 2wherein, the arcuate return portions curve away from the bend lineproximate the opposite ends and curve back toward the bend line atdistal ends of the return portions.
 4. The sheet of material as definedin claim 3 wherein, the arcuate return portions have a length dimensionalong the bend line equal to at least about 2 times the thicknessdimension of the sheet of material.
 5. The sheet of material as definedin claim 3 wherein, the arcuate return portions each have a lengthdimension along the bend line equal to at least about 20 percent of theoverall length of the bend-inducing structure along the bend line. 6.The sheet of material as defined in claim 4 wherein, the arcuate returnportions have a radius of curvature section over a majority of theirlength of at least 2 times the thickness dimension of the sheet ofmaterial.
 7. The sheet of material as defined in claim 4 wherein, thearcuate return portions have a radius of curvature section over amajority of their length of at least 3 times the thickness dimension ofthe sheet of material.
 8. The sheet of material as defined in claim 2wherein, the bend-inducing structure is an arc having a convex sidefacing and extending along the bend line, and the opposite ends of thearc transition to the return portions along arcs having a radius ofcurvature of between about 0.1 to about 1.0 times the thicknessdimension of the sheet of material.
 9. The sheet of material as definedin claim 2 wherein, the transverse dimension of the bend-inducingstructure from the bend line is less than about 20 percent of theoverall length dimension of the bend-inducing structure.
 10. The sheetof material as defined in claim 2 wherein, the bend-inducing structuresare formed by a plurality of connected arcuate sections that aresymmetrical about a transverse center line of the bend-inducingstructure perpendicular to the bend line, and the opposite ends of thebend-inducing structure have radii of curvatures less than the radii ofcurvature of the return portions.
 11. The sheet of material as definedin claim 10 wherein, the radii of curvature of the return portions areat least about 5 times the radii of curvature of the opposite ends ofthe bend-inducing structure.
 12. The sheet of material as defined inclaim 2 wherein, each return portion has a length equal to between about2 to about 4 times the thickness dimension of the sheet of material anda radius of curvature between about 2 and about 4 times the thicknessdimension of the sheet of material.
 13. The sheet of material as definedin claim 12 wherein, each end portion has a radius of curvature notgreater than approximately the thickness dimension of the sheet ofmaterial.
 14. The sheet of material as defined in claim 4 wherein, thebend-inducing structures are configured and positioned to produceedge-to-face engagement of the sheet of material on opposite sides ofthe bend-inducing structures during bending.
 15. A sheet of materialformed for bending along a bend line comprising: a sheet of materialhaving a plurality of bend-inducing slits positioned in longitudinallystaggered relation on alternating sides of the bend line and configuredto produce edge-to-face contact of the sheet of material on oppositesides of the slits during bending, the slits each being arcuate withconvex sides closest to the bend line and having arcuate return portionsat opposite ends of the slits extending back along the slits toward theother return portion, and the arcuate return portions having a lengthdimension and radius of curvature reducing stress concentrations. 16.The sheet of material as defined in claim 15 wherein, the arcuate returnportions have chords oriented substantially parallel to the bend line.17. The sheet of material as defined in claim 16 wherein, the radii ofcurvature of the arcuate return portions are at least about 2 times athickness dimension of the sheet of material.
 18. The sheet of materialas defined in claim 17 wherein, the radii of curvature of the returnportions are between about 2 and about 4 times the thickness dimensionof the sheet of material.
 19. The sheet of material as defined in claim15 wherein, the sheet of material is bent along the bend line.
 20. Thesheet of material as defined in claim 19 wherein, the sheet of materialis bent along the bend line into a three-dimension structure suitablefor loading transversely to the bend line.
 21. A method of increasingthe fatigue resistance of a structure formed by bending a sheet ofmaterial along a bend line having by a plurality of bend-inducingstructures positioned and configured to cause bending along the bendline, the method comprising the step of: forming the bend-inducingstructures to extend along the bend line and have arcuate returnportions extending away from opposite ends of the bend-inducingstructures and back along the bend-inducing structures toward the otherreturn portion, the return portions having a length dimension along thebend line and a radius of curvature selected to be sufficiently large tosignificantly reduce stress concentration and significantly increaseresistance to fatigue upon cyclical loading of the bent sheet ofmaterial the transverse to the bend line.
 22. The method as defined inclaim 21 wherein, the step of forming the bend-inducing structures witharcuate return portions is accomplished by forming the arcuate returnportions to have a radius of curvature between about 2 and about 4 timesthe thickness dimensions of the sheet of material.
 23. The method asdefined in claim 22 wherein, during the forming steps, forming thebend-inducing structures as one of slits, grooves and steps in the sheetof material.
 24. The method as defined in claim 23 wherein, during theforming steps, forming the bend-inducing structures with a configurationproducing edge-to-face engagement of the sheet of material on oppositesides of the bend-inducing structures during bending.
 25. A method ofpreparing a sheet of material for bending along a bend line into athree-dimensional structure and subsequent loading of the structuretransverse to the bend line, comprising the steps of: forming aplurality of bend-inducing structures along the bend line, thebend-inducing structures being at least one of slits, grooves and stepsin the sheet of material positioned proximate the bend line; and duringthe forming step, forming the bend-inducing structures with returnportions extending from opposite ends of the bend-inducing structuresaway from the bend line and back along the bend-inducing structurestoward the other return portion with each return portion having a lengthdimension along the bend line sufficient to substantially increase theresistance to fatigue failure when the structure undergoes transverseloading.
 26. The method as defined in claim 25 wherein, during theforming step, forming the arcuate return portions with chords orientedsubstantially parallel to the bend line.
 27. The method as defined inclaim 26 wherein, during the forming step, forming the arcuate returnportions with a radius of curvature at least equal to about 2 times thethickness dimension of the sheet of material.
 28. The method as definedin claim 27, and the step of: after the forming step, bending the sheetof material into a three-dimensional structure.
 29. The method asdefined in claim 28, and the step of: after the bending step, loadingthe three-dimensional structure transversely to the bend line.
 30. Amethod of forming a structure from a sheet of material to resistcyclical loading comprising the steps of: forming a sheet of materialwith a plurality of bend-inducing structures configured and positionedalong a bend line to produce bending of the sheet of material along thebend line; bending the sheet of material along the bend line to producea three-dimensional bent structure; and during the forming step formingeach bend-inducing structure as a continuous arcuate slit havingopposite ends curving away from the bend line and having arcuate returnportions extending from the opposite ends and curving back along theslit toward the other return portion with distal ends of the returnportions curving back toward the bend line.
 31. The method as defined inclaim 30, and the step of: after the bending step, cyclically loadingthe bent structure transversely of the bend line.
 32. The method asdefined in claim 31 wherein, the step of forming each bend-inducingstructure is accomplished by forming the arcuate return portions with achord length extending along the bend line by at least about 20 percentof the length of the bend-inducing structure along the bend line.